A bicycle is normally provided with a rear derailleur associated with the sprocket set, which consists of a set of coaxial toothed wheels (sprockets), with different diameters and numbers of teeth, integral with the hub of the rear wheel.
A bicycle is typically also provided with a front derailleur associated with the crankset, which consists of a set of toothed wheels (toothed crowns) with different diameters and numbers of teeth, associated with a bottom bracket axle driven into rotation by a pair of pedals. In less expensive bicycles, only one toothed wheel is associated with the bottom bracket axle and there is the rear derailleur only.
A transmission chain for transmitting the pedaling motion into the motion of the rear wheel extends in a closed loop between the sprocket set and the crankset.
The rear derailleur and the front derailleur—if present—engage the transmission chain, displacing it on toothed wheels with different diameters and numbers of teeth, so as to obtain different gear ratios.
By convention, one speaks of downward gearshifting when the chain shifts from a larger diameter toothed wheel to a smaller diameter toothed wheel, of upward gearshifting when the chain shifts from a smaller diameter toothed wheel to a larger diameter toothed wheel. In this regard it should be noted that in a front gearshift group, downward gearshifting corresponds to the passage to a lower gear ratio and upward gearshifting corresponds to the passage to a higher gear ratio; vice versa in a rear gearshift group, downward gearshifting corresponds to the passage to a higher gear ratio and upward gearshifting corresponds to the passage to a lower gear ratio.
The displacement in the two directions of a derailleur is obtained through a control device mounted so as to be easily handled by the cyclist, namely normally on the handlebars, near to the handgrips where there is also the brake lever to control the brake of the front and rear wheel, respectively. Control devices that allow driving both a derailleur in the two directions and a brake are commonly known as integrated controls.
By convention, the control device of the front derailleur and the brake lever of the front wheel are located near to the left handgrip of the handlebars, and vice versa the control device of the rear derailleur and the brake lever of the rear wheel are located near to the right handgrip.
More specifically, in a mechanical gearshift, each derailleur is moved between the toothed wheels, in a first direction by a traction action exerted by a normally sheathed inextensible cable (commonly known as Bowden cable), in a second opposed direction by releasing the traction of the cable and/or by the elastic return action of a spring provided in the derailleur itself.
Normally, but not necessarily, the direction in which the displacement is caused by the release of the traction of the cable and/or by the return spring is that of downward gearshifting; vice versa, the traction action of the traction cable normally occurs in the direction of upward gearshifting, in which the chain is displaced from a smaller diameter wheel to a larger diameter wheel.
The traction cable extends along the bicycle frame up to the control device. In the control device, the traction cable is traction- or release-actuated through winding and unwinding on a rotor element, commonly known as cable-winding bush, the rotation of which is controlled by the cyclist through suitable manual actuation means.
Typically, the manual actuation means comprise a pair of levers, a pair of buttons, a button and a lever, or a bidirectional lever.